Using Slant Fin Heat Loss Calculator

Crissie

Trying to use this app to size the heat load on a room by room basis. Does anyone know:
1. What is a cold partition wall, they list exposed walls separately, so is a cold partition an interior wall? Are these important to list?
2. For rooms with only interior doors, is it important to include those?
3. I am using an outdoor temperature of 1 degree here in Chicago area for heat load. Is that correct?

Does anyone know where I could get an app to calculate cooling loads?
Thanks!!!

CMadatMe

A cold patrician wall is a wall that separates a conditioned space from an unconditioned space. As example, a kitchen wall that separates it from a garage that is not heated. Interior doors are not calculated in a loss. You must have a difference in Temp indoor from Temp outdoor to have a panel loss. As for a design Temp, I'd use -10 as Jan/Feb will drop temps below 0.

KC_Jones

The lowest design temp I can find in the entire state of Illinois in -4, don’t use -10.

Do a web search for design temp and your zip code and it should pop up, or check this link and find your closest location.  That’s not the only resource, just a simple easy to read one I reference on occasion.

https://www.energystar.gov/sites/default/files/asset/document/Design%20Temperature%20Limit%20Reference%20Guide%20%282019%20Ed%29%20-%20ENERGY%20STAR%20Certified%20Homes_Rev10.pdf

CMadatMe

KC_Jones said:

The lowest design temp I can find in the entire state of Illinois in -4, don’t use -10.

Do a web search for design temp and your zip code and it should pop up, or check this link and find your closest location.  That’s not the only resource, just a simple easy to read one I reference on occasion.

https://www.energystar.gov/sites/default/files/asset/document/Design%20Temperature%20Limit%20Reference%20Guide%20%282019%20Ed%29%20-%20ENERGY%20STAR%20Certified%20Homes_Rev10.pdf

Yes b/c 6 degrees will make and break your boiler and design choice vs just turning down water temp if need be. Not sure how it would matter if the boiler choice is condensing. Could you please educate me on how 6 degrees will effect a condensing boiler choice or even a cast iron choice?

EdTheHeaterMan

This illustration may help. Look at the Blue outline. Those are the walls that you count in the load calculation as exterior walls. the wall indicated by the Green line is the partition wall


In the 2 story illustration, you do not count the RED line because the temperature between the first floor and the second floor are supposed to be about the same temperature (compared to the outside temperature of -10° below) the "first floor level" floor may be over a basement, on a concrete slab, over a heated basement, or over an unheated basement or crawlspace. You will need to determine that for your
own home and calculate accordingly.

I hope this helps you

Yours truly,
Mr. Ed

Crissie

Crissie said:

Trying to use this app to size the heat load on a room by room basis. Does anyone know:
1. What is a cold partition wall, they list exposed walls separately, so is a cold partition an interior wall? Are these important to list?
2. For rooms with only interior doors, is it important to include those?
3. I am using an outdoor temperature of 1 degree here in Chicago area for heat load. Is that correct?

Does anyone know where I could get an app to calculate cooling loads?
Thanks!!!

I used this for one open concept level are of my house, it seems to be estimating high. Does this sound accurate? Thanks!

Main Level is Living, Dining, Kitchen, Foyer, Mud Room / Garage Entrance:
Size Room: 33’ x 24’ - 792 sq ft
8 ft ceilings
118 sq ft of glass (lots of windows and door glass). All double pane, vinyl.
3 doors – total 57.78 sq ft
Below is unheated basement.
Above is attic with R50.
Exterior wall length (3 sides) = 68 ft. R15 with 1 inch foam board on 28 ft of it
Shares a wall with a garage. A cold Partition wall. 23 ft
Used .023 for infiltration factor because not a full three sides are exterior walls, more like 2.6 side are exterior.
Used 71 degrees for indoor and 1 degree temp for outdoor (app won’t do negative temps, 1 degree is close enough).
I got 31,311 BTU’s needed for hear, needs 55 ft of baseboard.

Crissie

Mr. Ed Thank you. This is how I used the app. It did not give an option of a unheated basement under my main level, so I used an unheated crawl. is that okay?

EdTheHeaterMan

I believe you are over sized by about 25% I just did a rough calculation and used the same factor for all the exterior walls including the cold partition wall. I came up with 27998 BTU. If I separate out the cold partition wall and added the additional foam insulation on the 28 ft section, that number will drop. I also used 80° difference between indoor and outdoor temps.

The infiltration factor for 6336 cu ft I used was 3/4 air changes per hour. Your may have used one that is higher than 1 air change per hour. This number is an indication of how tight the home is built. How windy is your area? when you get 40+ MPH winds does your house feel drafty? If so your air changes may be 2 air changes per hour or more and you calculation is correct. If you don't have that problem than you may want to check your infiltration selection.

Crissie

EdTheHeaterMan said:

I believe you are over sized by about 25% I just did a rough calculation and used the same factor for all the exterior walls including the cold partition wall. I came up with 27998 BTU. If I separate out the cold partition wall and added the additional foam insulation on the 28 ft section, that number will drop. I also used 80° difference between indoor and outdoor temps.

The infiltration factor for 6336 cu ft I used was 3/4 air changes per hour. Your may have used one that is higher than 1 air change per hour. This number is an indication of how tight the home is built. How windy is your area? when you get 40+ MPH winds does your house feel drafty? If so your air changes may be 2 air changes per hour or more and you calculation is correct. If you don't have that problem than you may want to check your infiltration selection.

I wonder if we are using the same app. It did not give me an option of a temp differential between the cold partition walls. What did you use for the unheated basement, an unheated crawl space? I had a blower door test done, house is very tight. 1 air exchange every 5.13 hrs.

ratio

CMadatMe said:

Yes b/c 6 degrees will make and break your boiler and design choice vs just turning down water temp if need be. Not sure how it would matter if the boiler choice is condensing. Could you please educate me on how 6 degrees will effect a condensing boiler choice or even a cast iron choice?

Sure, there's two reasons that jump out at me immediately. One, we're engineering this. We need as much accuracy as we can get, so we'll be able to make an informed decision WRT our guesses & fudge factors. There's going to be plenty of time to add them in (fudge factors) later on in the game to cover things we don't know for sure. No reason to do that here, especially since it's so easy to find. Two, once it comes time to actually select the equipment, we're certainly going to have to select something that's not exactly a perfect fit, in fact we're likely looking at something that's going to be twice the size of what we need, if the numbers I've seen floating around in this thread are any indication. If we go long on everything it'll of course push the final number up, possibly making an even larger unit look better than it should.

Crissie

CMadatMe and Ratio I just called Slant Fin. They were sold to Mestek. They said they haven't supported this app in years. Do you think that matters? They also said the BTU's output / ft for 160 degree design water is only 450 BTU's, not 580 or 600.
I am wondering if I should still use this app or if there is a better one available.
Thanks

ratio

It's the freeest one I know of. Support for it shouldn't matter past it'll either work or it won't.

EBEBRATT-Ed

@Crissie

I and many others on this forum have used the Slant Fin calculator. You can trust it

I have a Slant Finn CD disk if I can find it I will take your information posted above and post the results

EBEBRATT-Ed

@Crissie

I came up with 30,000 BTUs for the example above where you have 31,311

Not enough to quibble about. So I think you did it correctly

Crissie

EBEBRATT-Ed said:

@Crissie

I came up with 30,000 BTUs for the example above where you have 31,311

Not enough to quibble about. So I think you did it correctly

Great! What infiltration rate did you use, the app only gave me options and with my house being tight (1 exchange every 5 hrs) I think that might be throwing me off. BTW, I just did all rooms less a sunroom and total for house was 81,474 BTU's.